1. Field of the Invention
This invention relates generally to reduction in fretting between blades and disks in turbine engines, and specifically to reduction in fretting between titanium and titanium alloy compressor blade dovetails and titanium and titanium alloy compressor disks in the high pressure compressor portions of turbine engines.
2. Discussion of the Prior Art
Titanium and titanium alloys are used in the portion of aircraft engines to the fore or front portion of the engine because of their excellent mechanical properties, such as excellent strength, low density and favorable mechanical properties. However, the blades and the disks are usually separate parts that are fretted together, except in certain situations in which a blisk is used. The one disadvantage of separate titanium blades and titanium disks is that they rub against each other at the blade-to-disk attachment contact surfaces.
When two pieces of metallic material rub or slide against each other, frictional forces between the parts may result in damage to materials through the generation of heat, or through a variety of fatigue processes generally termed fretting or galling.
In certain aircraft engine designs, a titanium or titanium alloy compressor disk, also referred to as a compressor rotor, has an array of dovetail slots arranged around its outer periphery The compressor blades, also made of titanium or titanium alloy have corresponding dovetail bases to allow mate-up of the blade dovetail bases with the respective rotor dovetail slots so that the blade is retained within the dovetail slots. When the rotor is operating at normal operating speeds, centrifugal force causes the blades to move radially outward. The sides of the blade dovetail slide against the sides of the rotor slots.
Various approaches to solve the problem have been attempted in the region to reduce the damage due to fretting, with limited success. Copper-nickel-indium coating has been applied to the blade dovetail. While the coating has lowered the coefficient of friction between the blade dovetail and rotor dovetail slot, the reduction is not sufficient to eliminate fretting. Furthermore, once the coating wears off, in a few thousand cycles of engine operation, fretting once again becomes a problem.
Another solution to the problem has been to apply a dry film lubricant to the region between the blade dovetail and the rotor dovetail slot, such as is described in U.S. Pat. No. 5,356,545 assigned to the assignee of the present invention. While this invention has delayed the onset of fretting, it has not solved the problem. The dry film lubricant is displaced after about 2000 cycles or less of engine operation, and the normal processes leading to fretting occur after its loss.
Another solution has been to modify the area of highest stresses between the rotor dovetail slot and blade dovetail by undercutting the slot dovetail in the disk to remove disk material in the area where surface peak stressing (edge of contact) would otherwise occur, such as is described in U.S. Pat. No. 5,141,401, assigned to the assignee of the present invention. Once again, this solution has had varying amounts of success in reducing the time to crack initiation resulting from low cycle fatigue in that it is effective only while the wear does not approach the depth of the undercut, which in turn is limited by the dovetail size.
Despite all of the attempts to eliminate fretting, cracking resulting from such fretting continues to occur in high pressure compressor blade dovetails. Cracking has been observed on stage 3, 4 and 5 high pressure compressor blade dovetails at the upper edge of contact between the blades and the disk. The cracking occurs in engines that have experienced at least 4000 engine cycles, which corresponds to approximately 12,000 hours of engine operation. The problem to be solved is one of eliminating cracks induced by the forces generated between the dovetail of the blade and the dovetail slot of the rotor disk, thereby extending the operating life of the blades and hence the compressor assemblies.
What is needed is a new approach to reduce the fretting while simultaneously neutralizing degradation, thereby eliminating the onset of cracking so that engine life is not impacted by compressor problems in this area.